A hydraulic differential transmission utilizes a differential and a pair of hydraulically interconnected hydraulic units to control a speed relation between a drive input and a drive output. For many years, a hydraulic differential transmission has been used as a constant speed drive for a generator or alternator providing constant frequency electric power for an aircraft. The drive output must be maintained at a constant speed while the speed of the drive input from an aircraft engine varies. In use as a constant speed drive, it is difficult to make maximum use of the capability of the hydraulic units.
The hydraulic differential transmission can use either an output differential wherein the hydraulic units are driven from the drive input and the output differential functions to sum the speed of the drive input with the drive output of the hydraulic units or an input differential wherein the hydraulic units are driven by the drive output and the input differential sums the speed of the drive input and the speed of the drive output from the hydraulic units.
In the hydraulic differential transmission utilizing an output differential, one of the hydraulic units which is driven by the drive input must operate over the speed range of the drive input and, therefore, is able to drive the other hydraulic unit up to only a portion of its rated speed at minimum drive input speed.
In a hydraulic differential transmission utilizing an input differential, the pressure in the hydraulic units for a given mode is inversely proportional to the input speed which results in the hydraulic units operating at less than the normal pressure capabilities at the higher drive input speeds.
In either type of hydraulic differential transmission, the speed range can be reduced. One possibility for reducing the speed range is to achieve a change in drive ratios between the drive input speed and the hydraulic unit connected to the differential. This can be achieved by use of a shifting gearbox with resulting reduction in the speed range. However, the use of such a gearbox is undesirable, particularly in a constant speed drive for an aircraft because of the weight thereof and there is a discontinuity in the drive at the shift point between drive ratios.
The Zentz U.S. Pat. No. 3,733,924, owned by the assignee of this application, discloses a constant speed drive utilizing an output differential. The Reynolds et al U.S. Pat. No. 3,274,855, owned by the assignee of this application, discloses a constant speed drive utilizing an input differential for transmitting power from an aircraft engine to a generator to drive the latter at a constant speed regardless of variations of engine speed and load. The constant speed drive disclosed in the Reynolds et al patent is illustrative of a hydraulic differential transmission into which the invention disclosed in this application may be incorporated.
The invention to be described hereinafter embodies the incorporation of a second drive train between the input differential of the hydraulic differential transmission and one of the hydraulically interconnected hydraulic units to provide alternate drive ratios therebetween and with means for automatically shifting between drive trains, dependent upon the direction of rotation of said one hydraulic unit.